Re: [PATCH rdma-next 00/10] Enable relaxed ordering for ULPs

Tom Talpey
2021-04-09T17:49:27Z

On 4/9/2021 12:27 PM, Haakon Bugge wrote:
>
>
>> On 9 Apr 2021, at 17:32, Tom Talpey <tom@talpey.com> wrote:
>>
>> On 4/9/2021 10:45 AM, Chuck Lever III wrote:
>>>> On Apr 9, 2021, at 10:26 AM, Tom Talpey <tom@talpey.com> wrote:
>>>>
>>>> On 4/6/2021 7:49 AM, Jason Gunthorpe wrote:
>>>>> On Mon, Apr 05, 2021 at 11:42:31PM +0000, Chuck Lever III wrote:
>>>>>
>>>>>> We need to get a better idea what correctness testing has been done,
>>>>>> and whether positive correctness testing results can be replicated
>>>>>> on a variety of platforms.
>>>>> RO has been rolling out slowly on mlx5 over a few years and storage
>>>>> ULPs are the last to change. eg the mlx5 ethernet driver has had RO
>>>>> turned on for a long time, userspace HPC applications have been using
>>>>> it for a while now too.
>>>>
>>>> I'd love to see RO be used more, it was always something the RDMA
>>>> specs supported and carefully architected for. My only concern is
>>>> that it's difficult to get right, especially when the platforms
>>>> have been running strictly-ordered for so long. The ULPs need
>>>> testing, and a lot of it.
>>>>
>>>>> We know there are platforms with broken RO implementations (like
>>>>> Haswell) but the kernel is supposed to globally turn off RO on all
>>>>> those cases. I'd be a bit surprised if we discover any more from this
>>>>> series.
>>>>> On the other hand there are platforms that get huge speed ups from
>>>>> turning this on, AMD is one example, there are a bunch in the ARM
>>>>> world too.
>>>>
>>>> My belief is that the biggest risk is from situations where completions
>>>> are batched, and therefore polling is used to detect them without
>>>> interrupts (which explicitly). The RO pipeline will completely reorder
>>>> DMA writes, and consumers which infer ordering from memory contents may
>>>> break. This can even apply within the provider code, which may attempt
>>>> to poll WR and CQ structures, and be tripped up.
>>> You are referring specifically to RPC/RDMA depending on Receive
>>> completions to guarantee that previous RDMA Writes have been
>>> retired? Or is there a particular implementation practice in
>>> the Linux RPC/RDMA code that worries you?
>>
>> Nothing in the RPC/RDMA code, which is IMO correct. The worry, which
>> is hopefully unfounded, is that the RO pipeline might not have flushed
>> when a completion is posted *after* posting an interrupt.
>>
>> Something like this...
>>
>> RDMA Write arrives
>> PCIe RO Write for data
>> PCIe RO Write for data
>> ...
>> RDMA Write arrives
>> PCIe RO Write for data
>> ...
>> RDMA Send arrives
>> PCIe RO Write for receive data
>> PCIe RO Write for receive descriptor
>
> Do you mean the Write of the CQE? It has to be Strongly Ordered for a correct implementation. Then it will shure prior written RO date has global visibility when the CQE can be observed.

I wasn't aware that a strongly-ordered PCIe Write will ensure that
prior relaxed-ordered writes went first. If that's the case, I'm
fine with it - as long as the providers are correctly coded!!

>> PCIe interrupt (flushes RO pipeline for all three ops above)
>
> Before the interrupt, the HCA will write the EQE (Event Queue Entry). This has to be a Strongly Ordered write to "push" prior written CQEs so that when the EQE is observed, the prior writes of CQEs have global visibility.
>
> And the MSI-X write likewise, to avoid spurious interrupts.

Ok, and yes agreed the same principle would apply.

Is there any implication if a PCIe switch were present on the
motherboard? The switch is allowed to do some creative routing
if the operation is relaxed, correct?

Tom.

> Thxs, Håkon
>
>>
>> RPC/RDMA polls CQ
>> Reaps receive completion
>>
>> RDMA Send arrives
>> PCIe RO Write for receive data
>> PCIe RO write for receive descriptor
>> Does *not* interrupt, since CQ not armed
>>
>> RPC/RDMA continues to poll CQ
>> Reaps receive completion
>> PCIe RO writes not yet flushed
>> Processes incomplete in-memory data
>> Bzzzt
>>
>> Hopefully, the adapter performs a PCIe flushing read, or something
>> to avoid this when an interrupt is not generated. Alternatively, I'm
>> overly paranoid.
>>
>> Tom.
>>
>>>> The Mellanox adapter, itself, historically has strict in-order DMA
>>>> semantics, and while it's great to relax that, changing it by default
>>>> for all consumers is something to consider very cautiously.
>>>>
>>>>> Still, obviously people should test on the platforms they have.
>>>>
>>>> Yes, and "test" be taken seriously with focus on ULP data integrity.
>>>> Speedups will mean nothing if the data is ever damaged.
>>> I agree that data integrity comes first.
>>> Since I currently don't have facilities to test RO in my lab, the
>>> community will have to agree on a set of tests and expected results
>>> that specifically exercise the corner cases you are concerned about.
>>> --
>>> Chuck Lever
>